The present invention relates generally to a trigger holdoff system for an oscilloscope, and particularly to a trigger holdoff system for providing a stable trigger from a non-repetitive signal.
In conventional analog and digital oscilloscopes, an input signal is applied to a trigger generator, which generates a trigger signal in synchronism with the input signal. The trigger signal initiates a sweep generator to generate a sweep ramp signal in the analog oscilloscope or a time base control circuit to fill an acquisition memory in the digital oscilloscope. When monitoring a repetitive signal, a trigger holdoff circuit locks out the trigger signal from the sweep generator or the time base control circuit during a certain period of time to prevent premature starts of a new ramp excursion or a new acquisition cycle. Thus, the input signal is stably displayed on a screen or acquired into a memory in response to an identical trigger signal. However, the conventional trigger holdoff system functions properly only when the input signal is repetitive because the holdoff time period is fixed. That is, when the raw trigger signals are produced in groups of fixed numbers with variable time intervals therebetween, such as may be the output of a state machine, the timing of generation of a sweep ramp signal or an acquisition cycle is unstable.